(1)Field of the Invention
The present invention relates to a toner composition for developing an electrostatically charged image. More particularly, the patent invention relates to a toner composition in which the developing property, flowability and cleaning property are not influenced by environmental changes but can be maintained at high levels for a long time.
(2) Description of the Prior Art
In a copying machine or the like utilizing the electrophotographic process, various powdery toners formed by dispersing additives such as a colorant into a binder resin have been used for visualizing an electrostatically charged latent image formed on a photosensitive material comprising a photosensitive layer containing an inorganic or organic photoconductive substance according to the dry development method.
In this electrophotographic process, an electrostatic latent image formed on the above-mentioned photosensitive material by the charging and light exposure is developed by the above-mentioned toner, and the formed toner image corresponding to the electrostatic latent image is transferred onto a support such as a transfer paper and simultaneously, the toner image is fixed to the support by fixing means such as a heating roller or pressing roller to obtain a print. After the toner image has been transferred onto the support, the toner left on the photosensitive material is scraped out by a cleaning blade to remove the toner left on the photosensitive material and effect the cleaning.
In order to form good images in the above-mentioned system, the toner is required to have such a good charging property and a good developing property as not causing fogging or scattering of the toner and furthermore, at the cleaning step conducted after the transfer of the image, the toner is required to have such a property that the toner should not be left on the photosensitive material. Thus, the toner is required to have various characteristics at the respective steps.
Accordingly, a method for improving the charge stability and cleaning property by using a toner composition formed by adding a fine powder of an acrylic polymer to a toner has recently been proposed by Japanese Unexamined Patent Publication No. 60-186851, etc.
Furthermore, Japanese Unexamined Patent Publication No. 60-186854 proposes a toner composition comprising a fine powder of an acrylic polymer obtained by soap-free polymerization as a toner excellent in the resistance to environmental changes.
When a fine powder of an acrylic polymer is prepared by soap-free polymerization, particles having a relatively low content of a hydrophilic substance or a hydrophilic group can be synthesized, but since a water-soluble initiator is used for the polymerization, fragments of the initiator are left on the surfaces of the particles and no sufficient hydrophobicity is attained. Accordingly, if an image is formed by using a toner composition obtained by incorporating fine particles obtained by the soap-free polymerization into toner particles, an image having an excellent image can be formed at the initial stage and in a good environment, but with the lapse of time or in a high-humidity condition, insufficient cleaning, fogging of the image and reduction of the image density are caused, and it often happens that the image quality is drastically degraded. A method for dropping and supplying a toner to a toner-stirring zone from a hopper by rotation of a sponge roller or the like is often adopted. If a toner composition comprising fine particles of an acrylic polymer is used in this method, it often happens that the toner composition does not fall from the supplying roller, presumably because of a poor flowability, and in an extreme case, it becomes impossible to supply the toner composition. In a process unit generally adopted in recent years for attaining a maintenance-free effect (a developing zone, a photosensitive material, a cleaning zone and the like are integrated and the unit is discarded when the developer or photosensitive material becomes ineffective), a developer is charged in a narrow space of the developing zone, and if the developer is stored for a long time or allowed to stand still in the unused state, blocking is often caused in the toner by the weight thereof or changes of the temperature and humidity. In the case where the toner is supplied into a developing device from a conventional toner cartridge or toner holder, even if blocking is caused, the toner can be disintegrated by imparting a vibration or shock to the toner. However, it is not permissible to shake the process unit or give a shock to the process unit, because other mechanisms or members are adversely influenced. Therefore, the toner composition is required to have further improved moisture resistance and blocking resistance.